In general, attenuators adjust a wave form in an electronic circuit. FIG. 1 shows a circuit diagram for a prior art T-shaped attenuator 1. The T-shaped attenuator 1 includes a resistor 3 on a line 2, a resistor 4 in series with the resistor 3, and a grounded resistor 5 between the resistors 3 and 4. The resistors 3 and 4 have a variable resistance value, and their resistance values change in the same manner. On the other hand, the resistor 5 has a fixed resistance value.
Referring to FIG. 2, a circuit diagram illustrates another prior art attenuator. In general, the arrangement of resistors 6, 7 and 8 is in a T-shape as shown in the above prior art attenuator of FIG. 1. However, the resistors 6 and 7 are fixed in their resistance while the resistor 8 is variable.
In either of the above two prior art attenuators, the following relationships as shown in Equations (1) and (2) are held true among the resistances R1Ω for the resistor 3 or 6, the resistance R2Ω for the resistor 4 or 7, the resistance R3Ω for the resistor 5 or 8, the attenuation L dB and the impedance Z Ω:R1=R2=(1−10−L/20)Z/(1+10−L/20)  (1)R3=(2×10 −l/20)Z/(1−10L/10)  (2)As shown above in Equations (1) and (2), the attenuation L is changed by altering the R1, R2 or R3 value. The resistance values R1, R2 and R3, the impedance value Z and the attenuation value L are also interrelated. For example, in FIG. 1, when the attenuation L is changed by adjusting the resistance values in R1=R2 of Equation (1), the attenuation L in Equation (2) also changes. With the fixed resistance value R3, the impedance Z changes as the resistance values R1 and R2 are adjusted. In other words, the impedance is unmatched, and a desired attenuation amount is no longer obtained for the adjacent circuits.
For the above reason, it remains to be desired to have an attenuator that enables to vary the attenuation without influencing the impedance.